On 08May2022 22:48, Marco Sulla <marco.sulla.pyt...@gmail.com> wrote: >On Sun, 8 May 2022 at 22:34, Barry <ba...@barrys-emacs.org> wrote: >> >> In text mode you can only seek to a value return from f.tell() >> >> otherwise the behaviour is undefined. >> > >> > Why? I don't see any recommendation about it in the docs: >> > https://docs.python.org/3/library/io.html#io.IOBase.seek >> >> What does adding 1 to a pos mean? >> If it’s binary it mean 1 byte further down the file but in text mode it may >> need to >> move the point 1, 2 or 3 bytes down the file. > >Emh. I re-quote > >seek(offset, whence=SEEK_SET) >Change the stream position to the given byte offset. > >And so on. No mention of differences between text and binary mode.
You're looking at IOBase, the _binary_ basis of low level common file I/O. Compare with: https://docs.python.org/3/library/io.html#io.TextIOBase.seek The positions are "opaque numbers", which means you should not ascribe any deeper meaning to them except that they represent a point in the file. It clearly says "offset must either be a number returned by TextIOBase.tell(), or zero. Any other offset value produces undefined behaviour." The point here is that text is a very different thing. Because you cannot seek to an absolute number of characters in an encoding with variable sized characters. _If_ you did a seek to an arbitrary number you can end up in the middle of some character. And there are encodings where you cannot inspect the data to find a character boundary in the byte stream. Reading text files backwards is not a well defined thing without additional criteria: - knowing the text file actually ended on a character boundary - knowing how to find a character boundary Cheers, Cameron Simpson <c...@cskk.id.au> -- https://mail.python.org/mailman/listinfo/python-list
